Heretofore, an image sensor has been proposed, which is capable of signal detection at a high S/N ratio while removing the influence of ambient light (see, Koji Yamamoto, “Furiwake Tenso Hoshiki Oyobi Bubun Ryoiki Kosoku Yomidashi Hoshiki ni yoru Hencho Hikari Seibun Kenshutsu Kano na CMOS Image Sensor ni Kansuru Kenkyu (Research in Relation to CMOS Image Sensors Capable of Modulated Light Component Detection by Allocated Transfer and Subregion High Speed Readout Methods),” Graduate School of Materials Science, Nara Institute of Science and Technology, March 2006, hereinafter referred to as Document 1). The image sensor disclosed in Document 1 detects modulated light components using a charge allocation transfer method. However, the pixel circuitry thereof is of a structure in which two normal PG (photogate) method image sensors are used in combination and assembled together with a common PG as a light receiving element. Such an image sensor includes a structure in which there are two transfer gates sandwiching one light receiving element therebetween, and signal charge accumulating units are disposed on respective outer sides of the transfer gates. Further, according to Document 1, for increasing sensitivity, a structure is proposed in which the PG percentage formed by polysilicon is reduced. More specifically, an example is disclosed in which the PG is formed in a comb-like shape.
Further, as an exemplary application of an image sensor, a distance measuring system is known for measuring in a non-contact manner the distance to an object. Such a distance measuring system utilizes a time-of-flight (TOF) method. The TOF method operates by irradiating light with respect to an object, measuring a time period from irradiation of the light to impingement of the light on an object and until the light rebounds and is returned back, and then measuring the distance to the object based on this period and the speed of light (see, Ryohei Miyagawa and Takeo Kanade, “CCD-Based Range-Finding Sensor,” IEEE Transactions on Electron Devices, Vol. 44, No. 10, October 1997, pp. 1648 through 1652, hereinafter referred to as Document 2).
According to Document 2, there is disclosed in detail the irradiation timing of pulsed light in a distance measuring system, and the operational timing of two light receiving elements. More specifically, an irradiation timing and an irradiation stop timing of pulsed light are repeated at the same length (i.e., the light emitting element is driven at a duty ratio of 50%), and charges are transferred alternately in two directions in synchronism with irradiation and non-irradiation of the pulsed light (see FIG. 1 of Document 2). In addition, a period over which the pulsed light is reflected by an object and returned is measured based on the phase difference of two output voltages.